On their voyage back to civilization, the space miners onboard the Nostromo investigate an alien distress signal and inadvertantly become targets for an extraterrestrial lifeform with lethal survival instincts.
Much has been made of its gothic horror influences, but the tagline “In space, no one can hear you scream.”, can be seen as a call out to finding yourself in the middle of nowhere, the great expanse of the American West.
The Nostromo was a stagecoach bringing gold back to the East. The alien was an Apache. The main conflict wasn’t between the crew and the alien, but between the crew and the corporation.
Based in part on a story which later became a part of the novel, The Voyage of the Space Beagle, where the alien is literally described as a red-skinned devil.